Asdex Upgrade: Paving the Way for a Fusion Power Plant

Asdex Upgrade Plasma Vessel

The plasma vessel from Asdex Upgrade. At the bottom you can see the divertor’s baffle plates. Credit: © Volker Rohde

On March 21, 1991, the Asdex Upgrade experimental facility at the Max Planck Institute for Plasma Physics in Garching generated the first plasma.

For 30 years, the Asdex Upgrade has been paving the way for a fusion power plant that generates climate-neutral energy. The tokamak fusion plant was repeatedly expanded and improved during this time. Not least for this reason, it provides numerous insights that are incorporated into the design and operation of other fusion plants. For example, the Asdex Upgrade team has developed scenarios for the operation of the Jet test plant in the UK and the Iter test plant in France as well as forecasts for a planned demonstration power plant. A conversion planned for mid-2022 is intended to prepare the plant for the future.

The goal of fusion research is to develop a climate- and environment-friendly power plant. Like the sun, its purpose is to derive energy from the fusion of atomic nuclei. The fuel for this is an extremely thin, ionized hydrogen gas – a plasma. To ignite the fusion fire, the plasma must be enclosed in magnetic fields almost without contact and heated to over 100 million degrees.

In order to regulate the interaction between the hot fuel and the surrounding walls, scientists at the Max Planck Institute for Plasma Physics have equipped the Asdex Upgrade with a divertor, which has given the plant its name: Axial symmetric divertor experiment. Through an additional magnetic field, the divertor field removes impurities from the plasma and improves its thermal insulation.

Asdex Upgrade Plasma

View into the plasma of Asdex Upgrade. The edge of the plasma is directed onto the robust divertor plates at the bottom of the vessel. Credit; © MPI for Plasma Physics

However, in contrast to its predecessor Asdex, the Asdex Upgrade, the divertor and important properties of the plasma, especially the density and the load on the walls, are more closely adapted to the conditions in a later power plant. Equipped with a powerful plasma heater and sophisticated measuring equipment for observing the plasma, the Asdex Upgrade can therefore be used to develop operating modes for a potential power plant. In 38,700 plasma discharges to date, the plant has answered essential research questions for the European joint experiment Jet and the international experimental reactor Iter as well as a planned demonstration power plant.

A tungsten wall for the plasma vessel

With the Asdex Upgrade, the researchers took a significant step towards a future fusion power plant when they clad the wall of the plasma vessel with tungsten instead of carbon. Carbon has considerable advantages for experimental plants. However, it is unsuitable for the operation of a power plant because it is too strongly eroded by the plasma and binds too much fuel to itself. Because of its high melting point, tungsten is well suited as a wall material – at least in principle. But the plasma cools down quickly because of even the smallest impurities in the tungsten atoms that are repeatedly released from the wall. After a lot of experimentation, the Asdex upgrade team has been able to deal with this problem.

Direct consequences of this success: In a major rebuild, the European joint experiment Jet received a tungsten divertor in 2011. The international experimental reactor Iter team decided to forego the initially planned experiments with a carbon divertor and go straight for tungsten. Tungsten is also the reference material for the demonstration power plant.

Injecting hydrogen prevents instabilities

In the interaction of the charged plasma particles with the confining magnetic field, various disturbances of the plasma confinement can occur. These include instabilities at the plasma edge or ELMs (edge localized modes). In the process, the edge plasma briefly loses its confinement and periodically throws plasma particles and energy outwards onto the vessel walls. While medium-sized plants such as the Asdex Upgrade are able to cope with this, the divertor in large plants such as Iter could become overloaded. In order to solve this problem, procedures to prevent instabilities were developed for the Asdex Upgrade. Sixteen small magnetic coils in the plasma vessel completely suppress the instability with their fields. A second method starts at the outermost plasma edge. If the right plasma shape can be set – via the magnetic field – while ensuring a sufficiently high particle density – by injecting hydrogen – ELMs cannot develop.

Ensuring continuous operation

Continuous operation is guaranteed by fusion plants of the tokamak type – such as the Asdex Upgrade, Jet, or Iter – which construct the magnetic cage with two superimposed magnetic fields: a ring-shaped field generated by external magnetic coils and the field of a current flowing in the plasma. By combining the magnetic fields, the field lines are twisted in such a way that they enclose the plasma. The plasma current is normally induced in a pulse-wise manner by a transformer coil in the plasma. Unlike the more complicated stellarators, the entire system operates in pulses – a shortcoming of the tokamaks.

Scientists at the Max Planck Institute for Plasma Physics are therefore investigating various methods of continuously generating the current in the plasma. For example, by injecting high-frequency waves or particle beams that drive an additional current in the plasma. They have thus succeeded in operating the system almost without a transformer – and for the first time in a machine with a practically-relevant metallic inner wall. If the Asdex Upgrade had not been equipped with normally conducting copper coils but rather superconducting magnetic coils (as was the case for Iter), this phase could have been extended for much longer – potentially up to continuous operation.

What will happen next

During the 30 years of operation of the Asdex Upgrade, the divertor shape has been changed and optimized several times. The researchers now want to go a step further and test a new divertor concept. Two additional magnetic coils on the roof of the plasma vessel are intended to fan out the divertor field so that the power from the plasma is distributed over a larger area. Assembly of the coils is scheduled to begin in mid-2022. Such expansions will also enable future investigations at the Garching tokamak to solve the problems of a future demonstration power plant. “In many ways, the Asdex Upgrade can be seen as a blueprint for a tokamak fusion power plant,” says Project Leader Arne Kallenbach. “Together with newly developed computer codes, the sample discharges developed over 30 years provide reliable information for a power plant.”

4 Comments on "Asdex Upgrade: Paving the Way for a Fusion Power Plant"

  1. … April 1, 2021, am ann aaa

  2. Unfortunately, they won’t be able to make it function; they will remain quickly without the necessary rare metals, as they are all used by Musk, to make… batteries 🤨

  3. Kathy carpenter | April 9, 2021 at 1:53 pm | Reply

    And the catastrophic environment possibilities are what and a million degrees where is that heat going to go it’s got to go somewhere I want to know exactly how this is going forward what are the critical failure of all or any parts of this is their waste is it immeiadatly unstoppable where is a million degrees of heat going into our atmosphere.time and time again we’ve gone ahead with nuclear energy no matter the fact once it reaches catastrophic meltdown remember Chernobyl and Japan we cant stop it cant cool it down to keep it from radiating the environment for that whole part of the country it’s in then theres BP oil in gulf of Mexico catastrophic failure killed off everything cause they went ahead with the operation without having a damn clue how to stop their broken pipe then theres the valley in calif burning gas pipe releasing gas like a volcano for what a year cause they didnt know how to stop it when it failed and many many more examples of humans ability to in their excitement neglect to take seriously the hazards these things could and have produced the passed over likelyhood of it all going horribly wrong ohh the chances are slight and they do it anyway and then we get to watch these brilliant minds stand there and scratch their heads without a clue what to do about it when it fails miserably wheres all that heat going to go what are they going to be able to do to stop any type of failure experimenting again with creating something this hazardous is unacceptable unless every small step of the process is brought to failure and stopped immediately with backup failsafes and physically stopped before you continue with the next step in the progress think and act on catastrophic failures more than computer generated the chances are low so move on wheres that million degrees going to go during process constant release into our environment or failure and it all goes into our atmosphere at once with failure I want to know about failure s than the energy end gain

  4. Free energy? Free for who not for consumers of electricity which they are making us more reliant on with electric vehicles. Yet solar is there. All they need do is put panel on every building in the world. That would generate some amount of electricity. That would be free? Within reason. And then this is about the environment we are told. So solar panels and plant millions of acres of trees. In the middle ages ie from 15th to 18th centuries we were running out of forest in europe to cut down for building buildings and ships. So we expanded into other parts of the world, S, America for example. It is those trees which contained carbon, and water from the atmosphere. So when you cut them down you release that into the atmosphere and water comes down as rain causing floods etc. So if we want to be eco friendly we dont need to protest causing more pollution we just need to plant trees and god knows we have enough land to do so. That will help alleviate our problems. We could easily plant 1 million acres of trees a year. Massive plains of America. If we plant 1 tree every 50 metres we could still have plains with trees on them. And in Africa it has been shown with planting trees, areas have cooled by a few degrees, ground doesn’t dry out it holds moisture, gives shade assisting both wildlife and people. So why don’t we do the simple thing first. Is this about environment or power and control and maybe the ultimate weapon? After all plasma weapons what damage could they do. And if we create a plasma power plant small enough could we power aircraft and ships by it never needing to refuel? Never needing to rearm or make bullets or shells? Unlimited power and unlimited weapons?

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